At the present time, there are a number of small molecular weight drugs which are either being used therapeutically or are currently being tested in antiviral chemotherapy. The majority of these drugs are derivatives of nucleosides which exert their antiviral effects by penetrating viral-infected mammalian cells and interfering inside the cell with nucleic acid synthesis. To date, the failures of antiviral chemotherapy can be attributed in part to a lack of selective toxicity in that both host and viral nucleic acid synthesis are inhibited by these antiviral agents. Because antiviral effects are a consequence of traversion of the mammalian cell membrane, clinical efficacy depends upon achieving effective antiviral concentrations at the site of infection; in particular, intracellular levels of the antiviral agent must be adequate (Lorian, Antibiotics in Laboratory Medicine, 2nd Edition, Williams and Wilkins Publishers, p. 359, 1986).
A much less popular approach to antiviral chemotherapy has been the use of peptides which by themselves are too large to penetrate mammalian cells but will react with the surface envelope of the virus and inhibit viral infectivity. In blocking the site on the virus which attaches to the plasma membrane receptor of the host mammalian cell, the peptides may irreversibly and directly inactivate virus particles.
Among the peptides tested for antiviral activity, both synthetic poly-L-lysine and poly-L-arginine have been shown to possess inhibitory activity against influenza, Newcastle disease, tobacco mosaic, poliomyelitis, and mumps viruses (Watson and Bloom, Proc. Soc. Exptl. Biol. Med., 81, 29, 1952; Stahmann, et al., J. Biol. Chem., 189, 45, 1951; Green and Stahman, Proc. Soc. Exptl. Biol. Med., 83, 641, 1953; Katchalski, et al., The Proteins, p. 564, 1964). However, basic polyamino acids of either lysine or arginine have been shown to be toxic in mammals (Rubini, et al., Proc. Soc. Exptl. Biol. Med., 82, 231, 1953; Mauersberger, et al., Exptl. Pathol (Jena), 13, 268, 1977; Takada, et al., J. Pharmaceut. Sci., 71, 1410, 1982) making them unacceptable for human use. More recently, two of six natural peptides isolated from polymorphonuclear leukocytes and rich in the amino acids, cysteine and arginine, have been shown to display inactivating capability against Herpes simplex Virus Type 1 and other selected envelope viruses (Lehrer, et al., J. Virol., 54, 467, 1985). Although these natural compounds would be favorably expected to exhibit minimal toxicity, they showed little peptide-mediated inactivation at pH 5 or 6 and viral inactivation was relatively slow and only effective when pHs were 7.4 or preferably higher. These peptides would therefore be of little or no use, for example, for prevention or treatment of Herpes viral mucosal infections in the mouth or vaginal cavity, or for topical treatment on the skin where the pH is acidic rather than alkaline. The mean pH of the saliva under the tongue when taken in 385 subjects was noted to be 5.97 (range 5.73-6.15) (Jenkins, The Physiology and Biochemistry of the Mouth, p. 301, 1978) while the vaginal pH has been reported to range from 3.0 to 6.1 (Karnaky, Am. J. Surg., 101, 456, 1961). The secretions that accumulate on the skin's surface have been determined to be weakly acidic, pH of 4.5 to 5.5 (Weissman, Drug Intell. Clin. Pharm., 8, 535, 1974). Azen (Biochem. Genet., 16, 79, 1978) studied certain naturally occurring histidine-rich peptides of human saliva which contain up to 30% L-histidine; however, Azen reported that he found no inhibitory effect of these peptides against either DNA or RNA viruses in tissue culture experiments.